Phylum: Charophyta   Family: Desmidiaceae

Cosmarium cyclicum P. Lundell 1871

A distinct shape that is not readily confused with any other desmid. In face-view it has a distinct crenate margin. It seems to have quite a large size-range (giving rise to numerous varieties in the past) but retains its identifiable shape.
Cell dimensions: L: 55-57 µm; B: 62-64 µm; Is: 21-24 µm; Th: c.32 µm; L/B. 0.89.
An Arctic-alpine species that prefers sub-atmophytic habitats and ephemeral waters. West & West (1905: 145) provide no records for the whole of Scotland. In the Outer Hebrides it has only be found from a single site.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.